Microwave selective mode isolator



Jan. 16, 1962 J. J-ROSTELNICK MICROWAVE SELECTIVE MODE ISOLATOR FiledOct. 21, 1959 HOL LOW PIPE MODE SOURCE SIGNAL R mo W WW ML PP SOURCESTRIP LINE MODE FIG. 4

INVENTORI By J. J. KOSTELN/CK fl// SIG/VAL POWER souncs PUMP POWERSOURCE ATTORNEY United States Patent 3,017,577 Patented Jan. 16, 19623,017,577 MICROWAVE SELECTIVE MODE ISOLATOR Joseph J. Kostelnick,Middlesex, N..l., assignor to Bell Telephone Laboratories, Incorporated,New York, N.Y., a corporation of New York Filed Oct. 21, 1959, Ser. No.847,825 9 Claims. (Cl. 330-66) This invention relates to electromagneticwave transmission systems and more particularly to devices havingnonreciprocal transmission loss properties for use in such systems.

The use of materials having gyromagnetic properties to obtain bothreciprocal and nonreciprocal effects in microwave transmission circuitsis widely known and has found numerous and varied applications intransmission systems of both the wave guide and the transmission linetypes. A comprehensive survey of the uses and characteristics ofgyromagnetically active devices appears in the Proceedings of the IRE,vol. 44, No. 10, October 1956. More recent developments are reported inthe quarterly publication entitled IRE Transactions on Microwave Theoryand Techniques.

Included among the new transmission devices which have found widespreaduse in the microwave art is the so-called isolator. An isolator may bedescribed as a circuit element in which electromagnetic wave energypropagating in one direction, designated the forward direction,experiences but slight transmission loss while wave energy propagatingin the opposite direction, designated the reverse direction, experiencestransmission loss to the extent required by the system.

At least four distinct classes of isolators are presently known. Theseinclude the Faraday rotation types, the gyromatic resonance types, thefield displacement types, and, more recently, the mode turbulence types.The latter is described in detail in the copending applications of H.Seidel, Serial No. 774,547, filed November 17, 1958, now US. Patent No.3,010,086, and Serial No. 60,439, filed October 4, 1960, now US. PatentNo. 3,010,084, the latter being a continuation-in-part of applicationsbearing Serial Nos. 774,496, and 774,548, both filed November 17, 1958,and since abandoned.

During the period of the development of gyromagnetic devices, bothhollow pipe type wave guide embodiments and multiple conductor or stripline guide embodiments have emerged in each one of the four classes ofisolator. In general the structures within each isolator class forhollow pipe systems differ from those for strip line or coaxial systems.In some applications different isolator types are required at differentlocations within the system. Recently system applications have arisen inwhich a plurality of noncoupled wave modes of differing types arepropagated within'a common volume. It is desirable in these applicationsthat isolation be selectively introduced into one mode or channelwithout affecting the other modes. Prior art isolator structures ingeneral aifect all mode channels present.

' Accordingly, it is an object of the present invention to introducenonreciprocal transmission loss selectively to one channel of a multiplechannel wave guide' system with out at the same time affectingtransmission in either direction in the other channel or channels.

One specific field in which the principles of the present invention havepotential application is that of solid state microwave amplification inwhich the amplification occurs as the result of the simultaneouspresence of a plurality of transmission modes within a region comprisingmagnetically polarized paramagnetic material. In such devices at leasttwo noncoupled wave modes or channels are simultaneously present, one ofwhich is often a hollow pipe mode and represents pump power, the otheror others of which is often a strip line mode and represents the signalto be amplified. As is the case of the majority of microwave devices,provision is made for preventing reflected signal power from returningtoward the generator. At the same time there are other considerationswhich allow relaxation of system specifications with respect toreflected pump power and eliminate any need for pump power isolation. Byplacing isolator means heretofore known within the dual channel portionsof the amplifiers, both signal and pump power were afiected.

Accordingly, it is a further object of the invention to isolate signalpower in a solid state amplifier without at the same time affecting pumppower.

Furthermore, in some dual channel devices including gyromagneticmaterial for some purpose other than isolation, an external magneticbiasing field is applied at an acute angle to the plane of the radiofrequency magnetic field loops of the propagating energy. When isolationby means of gyromagnetic action is introduced into such a device it isadvantageous to utilize the angularly related field for dual purposes.

Thus a further object of the invention is to utilize, for the purpose ofintroducing isolation, a magnetic biasing field whose spatialrelationship to a multiple channel guide is determined by considerationsother than those pertaining to isolation.

It is a more specific object of the invention to orient the fieldpattern of the channel to be isolated with respect to the external fieldto produce maximum nonreciprocal eifect.

A feature in the utilization of the invention in a systems applicationis its compactness, which results in part from the use of a singlemagnetic bias producing means for a plurality of different gyrornagneticoperations.

In accordance with the present invention a first energy source applyingelectromagnetic waves in a hollow pipe wave mode and a second energysource supplying electromagnetic waves in a strip line wave modesimultaneously excite a dual channel structure adapted to support bothmodes. Within the structure the strip line wave mode is diverted into aguiding section incapable of supporting the hollow pipe mode in whichsection isolation means are provided. The hollow pipe mode passesthrough a guiding section incapable of supporting the strip line modeand, beyond the section containing the isolator these modes again entera common propagation path and proceed to their respective loads.

According to one embodiment of the invention, the hollow pipe wave modeis supported in a conductively bounded guide section within which isdisposed a fiat center conductor supportive of the strip line wave mode.Along a central longitudinal portion of the dual channel section aconductive septum extends longitudinally to divide the hollow guide intotwo electrically isolated guide portions, one of which is supportive ofthe hollow pipe mode at its assigned frequency, the other of which isnonsupportive of the hollow pipe mode. The flat center conductor isnarrowed and diverted into the latter guide portion. Within the guideportion containing the strip, gyromagnetic material is appropriatelydisposed toproduce directional transmission loss as disclosed forexample in the above-mentioned Seidel application Serial No. 774,547,now U.S. Patent No. 3,010,086. Within the gyromagnetically loaded regionthe strip may be physically reoriented by physical twists in thevicinity of the septum ends to effect maximum coupling between the stripline Wave mode power and the gyromagnetic material, as dictated by theparticular spatial orientation of an external magnetic bias. At theendof the conductive sep tum the narrowed conductive strip widens to itsoriginal width in the enclosing hollow guide and both wave mode channelsagain propagate within a common volume.

The above and other objects and features of the present invention, itsnature and its various advantages will appear more fully uponconsideration of the specific illustra'tive embodiments shown in theaccompanying drawing and described in the following detailed descriptionthereof:

In the drawing:

FIG. 1 is a partially broken away perspective view of an isolator inaccordance with the invention;

FIG. 2 is a transverse cross sectional view of the isolator of FIG. 1;

FIG. 3 is a cross sectional view of an alternate isolator embodiment;and

FIG. 4 is a block diagram representation of a solid state amplifiersystem incorporating the invention.

Referring more particularly to FIG. 1, there is illustrated aconductively bounded wave energy transmission path 11 into which variousconductive and dielectric media are disposed. Wave guide 11 is ofrectangular transverse cross section in the region of its terminal endportions 12, 13 and is proportioned such that unequal transversedimensions a, b are sufiiciently large to enable guide 11 to supportelectromagnetic wave energy in the dominant TE mode within the operatingrange of frequencies. Extending longitudinally within guide 11, andspaced away from the conductive Walls thereof, is conductive strip 14,which is proportioned to support a strip line type wave mode confined intransverse extent by the walls of guide 11. Portions 16, 17 of strip 14which extend through terminal end portions 12, 13 of guide 11 have theirwider transverse dimension parallel to the broad walls of the externalguide. Extending longitudinally within a central portion of guide 11 isconductive septum 15 which extends in a plane perpendicular both to thebroad Walls of guide 14 and to the end portions 16, 17 of strip 14.Septum 15 divides guide 11 into two conductively separate guide portionsor channels 18, 19. The ends 20, 21 of septum 15 would be provided withmeans for matching the impedance of the dual channel portion of guide 11with that of the single channel portion. These matching means may takethe form of a quarter-wave transformer, a long physical taper, or anyother appropriate matching device. For the sake of clarity, theimpedance matching means have been omitted from FIG. 1. The transverseplacement of septum 15 across the wider dimension of guide 11 isselected such that the dimensions of each of the channels 18, 19 in adirection parallel to dimension a of guide 11 differ. As is well known,this particular transverse dimension of a rectangular wave guidedetermines the largest wavelength Wave energy which will be supported bythe guide. Wave energy of larger wavelength or lower frequency will notbe propagated in the guide; i.e., the guide will be cutofi. Septum 15divides guide 11 such that the cut-off determining dimension of channel18 is large enough for channel 18 to support Wave energy in the dominantTE mode at the frequency introduced into guide 11 while the cut-offdetermining dimension of channel 19 is small enough that channel 19 willnot support, or is cut-01f for, the TE mode wave energy supported byguide 11 and channel 18.

Conductive strip 14 is shaped to extend only through channel 19. Forreasons which will become more apparent hereinafter, the plane of thewidest face of strip 14 in channel 19 is generally different from thecorresponding plane of end portions 16, 17. That is, strip 14 undergoesa twist 22 of one rotational sense in the vicinity of the end of septum15 and a twist 23 of the opposite rotational sense but equal magnitudein the vicinity of the end 21 of the septum.

Distributed along one sidewall of channel 19 are segments of elements 24of material which is capable of introducing nonreciprocal transmissionloss to energy propagating therealong. These elements are spaced apartsufficiently to allow the surrounding material, which in the illustratedembodiment comprises air, to fill the regions between the segments. Thenumber of segments is dependent upon the elected spacing and the totallength of channel 19. In any event one surface of each of the elementsis preferably in close proximity to strip 14 in accordance with theisolation principles set out in the copending Seidel applications. As aspecific embodiment, elements 24 may comprise material which exhibitsgyromagnetic properties over a range of operating frequencies ofinterest, commonly designated gyromagnetic material. The termgyromagnetic material is employed in this specification in its acceptedsense as designating the class of magnetically polarizable materialshaving portions of the atoms thereof that are capable of exhibiting asignificant precessional motion at frequencies within the frequencyrange of interest under the combined influence of an external magneticpolarizing field and an orthogonally directed varying magnetic fieldcomponent.- This precessional motion is characterized as having anangular momentum, a gyroscopic moment, and a magnetic moment. Includedin this class of materials are ionized gaseous media, paramagneticmaterials, and ferromagnetic materials including the spinel ferrites andthe garnetlike yttrium iron compounds. One particular class ofgyromagnetic materials suitable for use as nonreciprocal elements 24 inthe present invention comprises an iron oxide combined with a quantityof bivalent metal such as nickel, magnesium, zinc, manganese or othersimilar material. As a specific example elements 24 may comprisemagnesium aluminum ferrite prepared in the manner described in UnitedStates Patent 2,748,353 which issued to C. L. Hogan on May 29, 1956. Asdisclosed in the above-mentioned copending applications of H. Seidel,this material has been found to operate successfully as a nonreciprocalmode turbulence attenuator structure in the presence of a magneticbiasing field applied externally and directed orthogonally to the highfrequency magnetic field components of a strip line type wave mode. Therelative orientations among the various elements and channels alreadydescribed may be more readily appreciated by reference to FIG. 2 of thedrawing.

FIG. 2 is a transverse cross sectional view of the isolator of FIG. 1taken at line 2-2. Thus channel 18 is seen to be of greater horizontaltransverse dimension than channel 19, while both strip 14 and elements24 appear in channel 19. Magnetic polarizing field H illustrated byarrow 25, is applied to elements 24 from a source, not shown, externalto guide 11. As shown in FIG. 2, H is applied at an angle a to the topand bottom walls of guide 11. Such an orientation is often encounteredfor example in multiple energy channel systems in which solid stateamplification is produced. Strip 14, in the vicinity of its entry intochannel 19 is given a physical twist of magnitude sufi'icient to orientthe plane of the strip normal to the direction of arrow 25. When theelements and field are so oriented, elements 24 produce most efficientisolation. The wall 28 itself of guide 11 in the vicinity of channel 19may extend in part perpendicular to the plane of rotated strip 14 asshown, or a conductive plate so oriented may merely be inserted within arectangular guide. In either case the loading material should becontiguous to the slanted conductive boundary to insure efficient modeturbulence isolator performance.

In the operation of the microwave isolator of FIGS. 1 and 2, guide 11 isexcited in a hollow pipe wave mode, the dominant TE wave mode forexample, at terminal end 12 by source 31. Strip 14 is excited by source32 in a strip line wave mode at the same terminal end. Both modechannels enter guide 11 and are propagated normally as uncoupled wavemodes for several wavelengths until the vicinity of septum 15 isapproached. Power on strip line 14 is then, by virture of the taper 26,di-

verted to one side of guide 11. Passing matched end portion 20 of septum15, power in the hollow pipe mode passes into and through channel 18while power in the strip line mode passes into and through channel 19.No power in the hollow pipe mode propagates in channel 1) .due to itsrestricted dimensions. Likewise no power in the strip mode propagates inchannel 18 due to the lack of proper mode supporting means. Withinchannel 19 the strip mode is spatially reoriented at twist 22 and passesthrough the gyromagnetic region with little or, preferably, noattenuation. Any reflected energy, propagating within channel 19 in adirection opposite to that of the primary signal is significantlyattenuated by elements 24. Thus little or no unwanted reflected stripmode power will reach the components preceding the isolator. Uponreorientation at twist 23 and emergence from channel 19, the strip linemode power expands spatially along taper 27 until it is again supportedby original width strip 14. Hollow pipe mode power expands beyond septumto fill guide 11 once more. No mixing of the power in the two modes hasoccurred, and only power in the strip line wave mode has been affectedby the isolator means. Subsequent operations upon the strip mode mayoccur or, as illustrated in FIG. 1, connection to respective loads 33,34 may be made.

FIG. 3 is a transverse cross sectional view of an alternate isolatorembodiment of the invention. In general, reference numerals from FIG. 2have been carried over to corresponding parts of FIG. 3 whereappropriate. In FIG. 3, the gyromagnetic element takes the form of asingle thin element 30' of gyromagnetic material. Filling the regionbetween the gyromagnetic element 30 and the boundary wall 28 isdielectric slab 31. In general, the dielectric element 31 should have athickness measured parallel to the plane of strip 14 of the order ofthree times that of the gyrornagnetic element, and should have adielectric constant of the order of 10. The composite structure of FIG.3 serves to attenuate power in the strip line wave mode propagating in adirection opposite to the desired direction by virtue of the magneticfield differential across element 30 produced by the presence ofdielectric slab 31.

Practical applications of an isolator in accordance with the inventionmay involve the isolation of a signal source from a load, which maycomprise an amplifier section, or the isolation of successive amplifiersections from each other. Thus, in FIG. 4, a typical solid stateamplifier application of the invention is illustrated in block form. InFIG. 4, signal power from source 41 and pump power from source 42 passthrough isolator 43 which precedes amplifier 44. In this mannerreflected signal power from the amplifier is prevented from reaching thesource. A second isolator section 45 is interposed between amplifiers44- and 46 to prevent reflections from amplifier 46 from reachingamplifier 44. The signal power and pump power from amplifier 46, whichmay then pass through successive amplifiers and isolators, eventuallyarrives at respective loads 48, 49, The entire system of isolator andamplifier sections shown in FIG. 4 need have only a single appliedexternal polarizing field H indicated as arrow 47. As set out above, dueto amplifier considerations this polarizing field may be angularlyrelated to the broad faces of both the signal supporting strip line andthe pump power supporting hollow pipe guide. Within the isolatorsections 43, 45, however, the twisted strip enables maximum isolatorperformance.

In all cases it is understood that the above-described arrangements aremerely illustrative of the many specific embodiments which can representapplications of the principles of the invention. Thus, while a specificillustrative embodiment of the novel isolator has been disclosed withreference to a solid state amplifier system, the device is not limitedto such an application. Additionally, applications of the invention arepossible in which the external biasing magnetic field, rather than beingrelated by an acute angle to the broad walls of the hollow pipe guide,is applied in a direction normal to these walls. It is evident thereforethat numerous and varied arrangements other than those illustrated canreadily be devised in accordance with the principles of this inventionby those skilled in the art without departing from the spirit and scopeof the invention.

What is claimed is:

1. In combination, a transmission path for electromagnetic wave energyadapted to support first and second independent wave modessimultaneously, means within said path for segregating said first wavemode into a first channel and said second wave mode into a secondchannel which is physically separate from said first channel, one ofsaid channels including means for attenuating wave energy propagating inone direction therethrough while freely transmitting wave energypropagating in the opposite direction therethrough, and means forrecombining said modes in said path.

2. In a solid state amplifier system, microwave signal power in a firstwave mode, microwave pump power in a second Wave mode different fromsaid first wave mode, means for propagating said first and said secondwave modes simultaneously in a region loaded with paramagnetic material,means for subjecting said region to a magnetic polarizing field which isapplied at an acute angle to the predominant electronic field lines ofsaid first mode in said region, and means for attenuating signal powerin one direction of propagation tothe exclusion of pump power,comprising a bounded guiding section having first and second transverseportions, said signal power wave mode being coupled in otsaid firstportion and said pump power wave mode being coupled into said secondportion, gyromagnetic material disposed within said first portion alongone boundary thereof in the presence of said magnetic polarizing field,and means for reorienting the field pattern of said signal wave mode inthe vicinity of said gyromagnetic material.

3. Apparatus according to claim 2 in which said first wave mode is astrip line wave mode, said second wave mode is a hollow pipe guide wavemode, and said reorienting means comprises a strip line section having atwisted center conductor.

4. In an electromagnetic Wave system, a transmission path comprising aconductively bounded enclosure having a rectangular transverse crosssection and having a conductive member extending within and spaced awayfrom the boundary walls of said enclosure, means for exciting saidconductive member in a strip line wave mode and said conductivelybounded enclosure in a hollow pipe wave mode of a given frequency, aconductive septum extending longitudinally within said enclosure over aportion of its length and dividing said enclosure transversely into afirst channel the transverse dimensions of which are less than thosenecessary to support said hollow pipe wave mode at said given frequency,and into a second channel the transverse dimensions of which aresuflicient to support said hollow pipe wave mode, said conductive memberextending solely through said first channel, means for directionallyattenuating the energy propagated along said member disposed within saidfirst channel adjacent said member, said means comprising a materialexhibiting gyromagnetic properties, and means for impressing a magneticbiasing field upon said gyromagnetic material.

5. Apparatus according to claim 4 in which said biasing field is appliedat an acute angle to the walls of said enclosure and said conductivemember is reoriented within said first channel to align the planedefined by its broader transverse dimension normal to the magnetic fluxlines of said biasing field.

6. A microwave isolator comprising a conductively bounded wave guidingstructure adapted to propagate energy of a given frequency, a flatconductive strip within and longitudinally coextensive with saidstructure having first, second, and third longitudinally successiveportions, a conductive septum normal to the plane of said strip in saidfirst and third portions extending within said structure coextensivelywith said second portion and positioned to divide said structure into afirst channel which is below cut-oif for energy of said given frequencyand a second channel which is above cut-off for said energy, said stripextending only into said first channel, means for impressing an externalmagnetic field on said structure, said strip being oriented such thatthe plane defined by its broader transverse dimension in said secondportion is normal to the direction of said field, and nonreciprocalmeans for attenuating energy supported by said strip disposed adjacentsaid strip in said second portion.

7. The combination according to claim 6 in which said external magneticfield is applied at an acute angle to the References Cited in the fileof this patent UNITED STATES PATENTS 2,907,959 Robertson Oct. 6, 1959

